Gaseous fuels are turning out to be a major fuel source for power generation via internal combustion engines, which includes reciprocating engines and gas turbine machinery. This is mainly because of the requirement for cleaner combustion and thereby lesser pollutants. The gaseous fuels include natural gas, liquefied petroleum gas, bio-gas, land fill gas and producer gas derived from biomass. The air-to-fuel ratio (A/F) and other properties of various gaseous fuels are summarized in the Table below:
TABLE 1Energy density & air-to-fuel ratios (A/F) of various fuel gasesLCV,A/FA/FEnergy,Energy,Fuel GasMJ/kg(mole)(Mass)MJ/kgMJ/N3Hydrogen1212.434.43.423.19Natural gas50.29.517.22.763.40(~100% Methane)Bio gas,17.65.76.12.473.05(~60% Methane,40% CO2)Propane46.523.815.62.805.50Butane45.530.915.42.777.18Producer Gas5.01.11.352.132.56LCV: Lower Calorific Value
It is evident from the Table 1 that the A/F required for natural gas is about 9.5:1 on volume basis. The same for bio-gas (60% methane and rest carbon dioxide) is about 6.1:1, similarly the A/F for landfill gas would fall in-between based on the methane content and other combustible constituents in the fuel gas. Whereas, the A/F required for producer gas containing about 18-20% H2 & CO each, 2% CH4, rest, inert is about 1.12:1. Since the A/F for producer gas is widely different compared to either natural gas or bio-gas, the commercially available gas carburettor meant for either natural gas/land fill gas/bio-gas is unsuitable for producer gas. The commercial gas carburettors for fossil fuel need fuel gas at high pressure (˜105 Pa (1 bar)), as the fuel is available under pressure. These carburettors are designed for the fossil fuel is whose energy density is higher than the biogas or producer gas, having implications on the sizes and designed consideration. These carburettors will have small passages for the same energy input compared with low energy content gas.
Typically, for producer gas whose energy content is about 1/10th that of any fossil fuel, requires 10 times the flow into the same fossil fuel carburettor, which is impossible due to issues related to pressure drops, etc a fluid dynamic issue. The producer gas generated in a gasifier is typically available at a pressure slightly above the ambient pressure (˜3000-4000 Pa).
Therefore if an engine fitted with natural gas carburettor were to be used for producer gas operation then it will lead to operation that at not optimal with respect to the performance and the delivered power by the engine would be very low. This therefore calls for a different carburettor to suit A/F requirement for producer gas fuel.
There have been many designs of carburettors for various fuel gases meant to be used in gas engines.